In noticing the animal of a univalve shell, the part which more readily
attracts the attention is the mantle, which covers the head of the
creature, something like a hood; it varies much in form and size in
different genera. The eyes, which in the sepia are amazingly large and
brilliant, are very minute in most of the other tribes, although they
are frequently visible, and would appear, from their formation, to be
of little use as organs of sight; indeed, it is supposed, that in the
snail they are devoted to the sense of smelling.
The organs of motion in the Mollusca, according to their different
form and position, give names to most of the orders; these consist of
muscular expansions of the body, by means of which the animal swims or
drags itself along the ground. The gills, or breathing apparatus, are
situated internally, and communicate with the air or water, by means of
a small canal opening outwardly. The mouth is usually concealed from
view when the creature is at rest; in some, this organ is furnished
with a hard substance, which supplies the place of teeth, while, in
others, it is in the form of a projecting tube. The greatest portion
of these creatures are produced in the water, the tribes that inhabit
that element, exceeding by far those that are to be found upon the land.
The Molluscous animals have been separated into the five following
Orders:—
1. HETEROPODA, (_with feet, or organs of motion,
not uniform in all species_.)
2. CEPHALOPODA, (_with feet, or organs of motion,
attached to the head_.)
3. TRACHELIPODA, (_with feet, or organs of motion,
attached to the neck, near the gills_.)
4. GASTEROPODA, (_with feet, or organs of motion,
attached to the stomach_.)
5. PTEROPODA, (_with feet, or organs of motion,
like wings_.)
_ORDER HETEROPODA._
The creatures belonging to this order, says Lamarck may be considered
as the first vestiges of the appearance of a series of marine animals,
intermediate in their formation between the fishes and the Cephalopods;
they are all natives of hot climates, and possess a body of a jelly-like
substance, and so transparent as to be seen with difficulty when
floating in the water. They do not all possess shells, and are less
known than they otherwise would be, on account of the great difficulty
there is in preserving them.